1. Field of the Invention
The present invention relates to a fixing apparatus applied to an image forming apparatus such as a copying machine, a printer and the like.
2. Related Background Art
As a conventional image heating and fixing apparatus for heating and melting an image borne on a recording material and for fixing it on the recording material, a heat roller fixing method has been widely used which has a heating roller kept at a predetermined temperature and a pressure roller with an elastic layer for pressing the heating roller, to thereby heating the recording material on which an unfixed toner image is formed while pinching and conveying the recording material by both the rollers.
However, for example, this heat roller fixing method suffers from the following disadvantages. Firstly, the method needs a rather long period of time for prohibiting the image forming operation until a predetermined temperature is reached, so-called wait time. Secondly, since a heat capacity is needed, a relatively large amount of electric power is needed. Thirdly, since the roller temperature of the rollers is high, a heat resistant special bearing is needed. Fourthly, it is necessary to take a structure such that a user's hand comes into direct contact with the roller, and therefore, a protective member therefor is needed to avoid danger. Fifthly, the recording member is wound around the roller due to the roller fixing temperature and the curvature of radius of the roller, which leads to a cause of generation of jam.
Therefore, in a film heating type fixing method proposed by the present applicant, it is possible to use a low heat capacity linear heating member in comparison with the above-described heat roller fixing method or any other well known thermal fixing systems such as a belt drive method, a flash fixing method, or an oven fixing method. Accordingly, the electric power saving (i.e., quick start property) is possible. Also, since the fixing point and the separation point can be set separately, the offset may be prevented. In addition, the proposed method may solve the other disadvantages inherent in the other systems employing other methods, and is effective.
For instance, the method, apparatus or the like disclosed in Japanese Patent Application Laid-Open No. 63-313182 belongs to the above-described proposal made by the present applicant. This method or system includes a thin heat resistant film (sheet), a moving drive means for this film, a heating member disposed and fixedly supported to one surface side while pinching the film, a pressure member for bringing the surface bearing a visualized image of a recording material to be image processed and fixed via the film into contact with the heating member, or the like.
The above-described film travels and moves at the same velocity in the forward direction together with the recording material, to be image processed and fixed, which is conveyed and introduced in between the film and the pressure member at least upon executing the image fixing. The recording material passes through a fixing nip portion formed (defined) by the pressure contact between the heating member and the pressure member while pinching the traveling and moving film so that the visualized image bearing surface of the recording material is heated by the heating member via the film and heat energy is applied to the visualized image to be softened and molten.
Subsequently, the film and the recording material are separated away. Alternatively, the film and the recording material are separated away from each other after a toner is cooled and solidified.
In such a film heating type fixing system, an electric supply heat generating layer having a direction that perpendicular to the moving and passing direction of the recording material in a longitudinal direction is provided. When the voltage is applied between electric supply electrodes of both terminal portions in the longitudinal direction of this electric supply heat generating layer, each portion in an effective entire length region of the heat generating layer generates heat with a certain heat generation amount per unit length. The effective entire length region of the heat generating layer has a length corresponding to a width (maximum width, maximum size width) of the maximum size recording material that may be fed and used in the image forming apparatus into which the fixing apparatus is assembled.
When the fixing operation is executed, the effective entire length region of the heat generating layer is energized to heat with a predetermined heat generation amount per unit length regardless of the extent of the size width of the used recording material.
However, the above-described conventional film heating type fixing system suffers from the following possible problems.
FIGS. 10A, 10B and 10C are views showing the fixing apparatus that is made according to the background art of the present invention.
As shown in FIG. 10A, a fixing nip portion A is formed by the pressure contact between a substantially flat heating member 61 and a roll-shaped pressure member 7 while pinching a cylindrical fixing film 65. As shown in FIG. 10B, a mechanical hysteresis is likely to be left on the side of the pressure member 7 in the recording material S passing through the fixing nip portion A. Namely, the recording material is likely to be discharged under a downward curl condition (a convex condition upwardly). In particular, this curl is likely to be appeared at a tip end of the recording material.
The image is fixed by using the heating member 61 having a low heat capacity to instantaneously apply the heat energy to the recording material S via the film 65. Therefore, as shown in FIG. 10C, in particular after the start of the apparatus, the temperature difference from the pressure member 7 is remarkable, the downward curl of the recording material S is remarkable, and in some cases, even the creases are generated in the recording material S.
Also, the generation of the curl or crease in the recording material S leads not only to a degradation in quality of the discharged recording material but also to lowered performance in stacking the discharged recording material and a feeding trouble in sheet feeding of double-sided sheet or in sheet feeding by ADF. This leads to less reliability of the apparatus itself.
Therefore, the present applicant proposes to take countermeasures against the curl of the recording material, in which, as shown in FIG. 11, a guide 80 on the side of the outlet of the nip portion is raised upwardly from below an extension line 81 of the nip portion.
However, the slant of this guide is relatively gentle and would not be sufficient for correcting the relatively large curl generated in the recording material, in particular, the large curl generated at the tip end of the recording material.